The present invention relates to fiber-optic communications networks, and more particularly, to pump systems for Raman amplifiers in fiber-optic communications networks.
Fiber-optic networks are used to support voice and data communications. In optical networks that use wavelength division multiplexing, multiple wavelengths of light are used to support multiple communications channels on a single fiber.
Optical amplifiers are used in fiber-optic networks to amplify optical signals. For example, optical amplifiers may be used to amplify optical data signals that have been subject to attenuation over fiber-optic links. A typical amplifier may include erbium-doped fiber coils that are pumped with diode lasers. Raman amplifiers have also been investigated. Discrete Raman amplifiers may use coils of fiber to provide Raman gain. Distributed Raman amplifiers provide gain in the transmission fiber spans that are used to carry optical data signals between network nodes.
Raman amplifiers may be pumped by linearly-polarized laser diodes. However, if a Raman amplifier is pumped with a single linearly-polarized laser diode, the Raman gain will exhibit a polarization dependence. With this arrangement, optical signals with different polarizations will be amplified by different amounts. Because polarization dependent gain is undesirable, Raman pumps often are formed using two linearly-polarized lasers that are combined using a polarization beam combiner. With this type of arrangement, the combined pumped signal is unpolarized and is suitable for pumping a Raman amplifier. However, this type of arrangement requires two Raman pumps and a polarization beam combiner.
Another approach for forming a polarization-independent Raman pump source involves using laser diode devices that are stabilized with external fiber gratings. In a typical pump of this type, a first polarization-maintaining fiber with a fiber grating is coupled to a multiple-quantum well device. The light exiting this first polarization-maintaining fiber is linearly polarized. To unpolarized this light, the first polarization-maintaining fiber is spliced to a second polarization-maintaining fiber. The first and second polarization-maintaining fibers are oriented so that their principle axes meet at a 45xc2x0 angle at the splice. With this arrangement, light that is launched from the first polarization-maintaining fiber into the second polarization-maintaining fiber is equally coupled into both the slow and fast principle axes of the second polarization-maintaining fiber.
The second polarization-maintaining fiber is typically a few meters in length. After the pump light passes through this length of fiber, the signals traveling along the slow and fast axes become spatially separated by a length that is greater than the coherence length of the multiple quantum well device. As a result, when the pump light exits the end of the second polarization-maintaining fiber, the light is unpolarized. This unpolarized light may be used to pump fiber in a Raman amplifier. However, it is difficult to form the 45xc2x0 splice between the first and second polarization-maintaining fibers.
It is therefore an object of the present invention to provide Raman amplifier pump systems that provide unpolarized light for Raman amplifiers.
It is also an object of the present invention to provide Raman amplifier pump systems in which a polarization-maintaining fiber is coupled directly to a multiple quantum well device at an orientation that allows light from the multiple quantum well device to be launched about equally into both the slow and fast axes of the fiber.
These and other objects of the invention are accomplished in accordance with the present invention by providing Raman amplifiers for amplifying signals on optical communications links. The communications links may support channels operating at one or more different wavelengths. The Raman amplifier equipment may be based on distributed or discrete Raman amplifiers.
The Raman amplifiers may be pumped using an unpolarized source of pump light. The unpolarized source may be formed using a polarization-maintaining fiber. Gain for the source may be provided by a semiconductor device such as a multiple quantum well device that provides linearly-polarized pump light. The polarization-maintaining fiber may be coupled directly to the multiple quantum well device at a 45xc2x0 orientation with respect to the polarization of the light emitted by the multiple quantum well device. This configuration allows light from the multiple quantum well device to be launched equally into both the slow and fast axes of the fiber. The polarization-maintaining fiber may have a fiber Bragg grating reflector for forming an external cavity laser based on the multiple quantum well device.
The grating on the polarization-maintaining fiber may be located a few meters from the multiple quantum well device. After the light from the multiple quantum well device has passed through this length of fiber, the signals traveling along the slow and fast axes become spatially separated by a length that is greater than the coherence length of the multiple quantum well device. As a result, when the pump light exits the end of the polarization-maintaining fiber, the light is unpolarized. This unpolarized light may be used to pump fiber in a Raman amplifier.